1. Field of Use
This invention relates generally to a controlled kinetics torque converter for transmitting power between a prime mover and a hoist drum in a mobile crane. In particular, the invention relates to apparatus and method for varying the density of the working fluid in the torque converter to regulate the capacity thereof and for keeping the working fluid above a predetermined pressure level to prevent unwanted cavitation.
2. Description of the Prior Art
In some mobile cranes, it is the practice to employ friction-type clutches between the prime mover and the hoist drum. However, clutch drives of this character introduce problems of control arising from order of magnitude variations of the coefficient of friction of the clutch surfaces under varying load and operating conditions. To avoid these problems, some prior art cranes employ a torque converter between the prime mover and the hoist drum and means are provided to adjust the angle of the torque converter stator blades to change the capacity of the torque converter while the prime mover operates at a constant speed. More specifically, for example, a three stage stationary housing torque converter comprises a bladed impeller mounted on an input shaft connected to the prime mover, a bladed turbine mounted on an output shaft connected to the hoist drum drive shaft, and stator blades which are mounted in the stationary housing and are angled to redirect a working medium such as oil from the impeller blades onto the turbine blades. Initially, the turbine will begin to rotate as the momentum of the oil provided by the impeller is absorbed by the turbine and the output shaft is accelerated. When the turbine is stopped, the torque converter is "stalled," and the oil flow is predominantly along the design path or torus in the housing of the torque converter. This helical oil flow path provides a speed reduction from input shaft to output shaft with a commensurate torque increase to provide conservation of work energy less viscous losses. As the turbine speed approaches impeller speed, the oil flow is predominantly in the same direction as the rotation of the turbine, and output torque drops to the level of input torque. The relationship between input and output torques and speeds is determined by the construction of the torque converter and by the density of the working fluid. The torque multiplication as a function of speed ratio, where speed ratio = (output speed/input speed), varies with the number of stator and turbine blades and the angles and shape of the individual blades. For maximum efficiency, the ideal torque converter working fluid would have minimum viscosity, to minimize losses, and maximum density since the action of the torque converter is basically the transfer of momentum.
In operating a crane, a given load, lifted by a given engine at a given speed through a given torque converter, will rise at a certain fixed speed. To control movement of a load with an engine at constant speed, the amount of power transmitted through the torque converter must be infinitely variable. To provide a range of control over the power transmitted through a torque converter, either the converter construction or the fluid density must be varied. As hereinbefore mentioned, some torque converter designs permit continuous adjustment of the angle or the effective area of the torque converter stator blades, and this provides for control over a limited range.
As hereinbefore mentioned, control is also possible if the density of the working fluid can be varied.
As is well known, a simple reduction of the amount of working fluid in the torque converter will not permit controllable power transmission. The partially filled torque converter will transmit less power, but the level of reduction will fluctuate. Cavitation will occur at a rate dependent upon input and output power. Draining oil from the torque converter will reduce output speed and increase cavitation which will lessen the power output further, thereby lessening output speed, increasing cavitation, and so forth. On the other hand, increasing the working fluid level will cause the opposite, self-exciting instability.
It is also well established that a hydrokinetic torque converter can be "unloaded" by reducing the internal fluid pressure of the working fluid and allowing cavitation to occur. This produces a reduction of output torque, input torque, and usually a loss of efficiency. The action is not smoothly controllable because the cavitation builds up and decreases with speed ratio change and with input power change and speed change. Generally, the effect is for the working fluid to cavitate more as the speed ratio decreases below the peak efficiency point and to cavitate more as input speed increases and as input power increases.
The control of most loads is not controllable by lowering the internal pressure of the working fluid. The load causing cavitation as pressure is reduced will slow to a lower speed ratio causing more cavitation which produces less torque resulting in an even lower speed ratio and so forth. As cavitation increases, less and less working fluid is available in the torque converter and cavitation increases uncontrollably. Input torque is reduced allowing the prime mover to speed up causing more cavitation and loss of load control. A pressure increase imposed on the working fluid will cause a self-regeneration sequence in the reverse order; the result being an uncontrolled "picking up" of the load.
The following patents exemplify hydrodynamic transmissions or fluid couplings wherein attempts are made to control or regulate those devices by controlling the working medium supplied thereto: U.S. Pat. Nos. 2,768,722; 3,382,959; 3,844,120; and 3,724,209.